Water-based maskant composition and chemical milling method using the same

A water-based maskant composition for chemical milling, which consists essentially of 100 parts by weight (as solids) of a latex constituent consisting of (a) 5-60 parts by weight (as solids) of a copolymer latex obtained by emulsion-polymerization of a monomer mixture consisting of 30-70% by weight of an aliphatic conjugated diene compound, 20-70% by weight of an aromatic vinyl compound, 0-10% by weight of an ethylenically unsaturated carboxylic acid and 0-50% by weight of a vinyl compound copolymerizable therewith and (b) 95-40 parts by weight (as solids) of a natural rubber latex;

• 10-500 parts by weight of talc;
• and 0.1-10 parts by weight of a thickener selected from the group consisting of polyvinyl alcohol, sodium salt of polycarboxylic acid, potassium salt of polycarboxylic acid, ammonium salt of polycarboxylic acid, cross-linking acrylic emulsion and carboxymethylcellulose, wherein the viscosity at 25.degree. C. is 500-5,000 cps as measured by BH type rotor at 20 rpm at a solids concentration of 50% by weight, the pH is 7-9, and the tensile strength and elongation of a film obtained from the composition are 50-150 kgf/cm.sup.2 and 100-800%, respectively, as measured according to JIS K 6301.